Skimming is the final top coat or finish coat of plaster that is applied either to either a backing coat, plasterboard or old and damaged existing plastered walls - to provide a nice smooth finish ready for painting.

The skim (finish) coat is the final coat and is responsible for smoothness. It is not designed to flatten so the background to which it is being applied needs to be flat.

Firming up

An uneven background can result in some areas having a greater thickness of plaster. The firming up process is influenced by the thickness of plaster. Progression through all the stages of skimming is in turn influenced by the firming up process. Therefore if one part of the wall is twice as thick as another it could take twice as long for it to firm up and therefore move through the stages and could result im an uneven finish.

Learning to Plaster

When you learn to plaster expect some problems but if you do nothing else then remember when you are applying the plaster to keep it flat.